Typically, a line-and-space structure is formed by etching when semiconductor devices are manufactured from a wafer serving as a substrate.
FIGS. 7A to 7D are examples showing processes of forming the line-and-space structure by the etching. Specifically, FIGS. 7A and 7B show the process of forming the line-and-space structure of a bottom anti-reflective coating (BARC), and FIGS. 7C and 7D show the process of forming the line-and-space structure of a photoresist film.
As shown in FIGS. 7A and 7B, a wafer includes a base film 72, a BARC film 73, and a photoresist film 74, which are successively stacked on a silicon base layer 71. The photoresist film 74 has a pattern structure that partially exposes the BARC film 73 and the exposed BARC film 73 is etched by using the photoresist film 74 as a mask. During the etching, the BARC film 73 is etched in the width direction (left/right direction in FIGS. 7A and 7B) as well as in the thickness direction (up/down direction in FIGS. 7A and 7B). As a result, a plurality of narrow lines 75 is formed from remnants of the BARC film 73 on the base film 72.
As shown in FIGS. 7C and 7D, a wafer includes a base film 77, an organic film 78, a silicon-containing anti-reflective coating (SiARC) film 79, and a photoresist film 80, which are stacked in that order on a silicon base layer 76. The photoresist film 80 has a predetermined pattern structure and only the photoresist film 80 is etched. During the etching, the photoresist film 80 is also etched both in the width and the thickness direction. As a result, a plurality of narrow lines 81 is formed from remnants of the photoresist film 80 on the SiARC film 79.
Recently, the semiconductor devices have become scaled down, which requires finer processing dimension and higher processing accuracy. For example, in the line-and-space structure formed by etching, a line width (a critical dimension (CD) value, and hereinafter referred to as a “CD value”) deviated from a desired value by only several nanometers, may result in non-acceptable semiconductor device performance. Accordingly, it is required to precisely control the CD value of the lines 75 or 81 in the etching.
However, the etching in FIGS. 7A and 7B is ended at the time when the BARC film 73 is partially etched and the base film 72 is exposed. The etching in FIGS. 7C and 7D is terminated if a preset etching period of time has lapsed. That is, the etchings are ended without measuring dimensions related to lines or the like. This makes it very difficult to control the accurate CD value of the lines 75 or 81.
Accordingly, there have been suggested methods of measuring the dimensions or the like related to lines by using reflected light of a white light beam and controlling the etching according to the measured result. For example, a feedback method and a feedforward method have been used. According to the feedback method, in a test wafer, a line-and-space structure for performing the measurement thereon is formed at a position, on which it is easy to perform the measurement, by the etching under a predetermined processing condition. Then, the CD value of the formed line-and-space structure is measured and the processing condition of the etching is adjusted according to the measured CD value. In the feedforward method, the dimensions or the like of pattern structure of a photoresist film of a wafer is measured before etching and the processing condition of the etching is changed according to the measured dimensions or the like.
Since, however, neither the feedback method nor the feedforward method directly measures the dimensions of the formed lines, there still remain problems in quality guarantee. Moreover, since both of the methods perform the measurement by using dedicated measuring devices, the throughput is lowered.
Further, there has been known a method, which monitors or measures the film thickness of a mask layer during the etching and ends the etching when the monitored film thickness becomes a preset thickness. (see, e.g., Japanese Patent Laid-open Application No. 2006-86168 and corresponding U.S. Pat. No. 7,514,277). In this method, the interference between reflection beams reflected from a surface of a mask layer and an interface of the mask layer and a silicon layer, which varies according to the film thickness of the mask layer, is utilized. Specifically, light is irradiated on a wafer and reflection beams reflected from the wafer are detected to evaluate the spectral reflectance (reflectance spectrum) of a plurality of wavelengths, and then measure the film thickness of the mask layer according to a preset calibration curve (reflectance spectrum).
To meet the requirement for the finer processing dimension, a plurality of lines having a same CD value is arranged with identical intervals therebetween in the fine line-and-space structure. Since, however, a space width and the CD value of each line are about tens of nanometers, the fine line-and-space structure forms a diffraction grating. In the diffraction grating, if a grating width corresponding to the CD value of lines is changed, a reflected beam brings about a phase shift and becomes a diffraction wave. Likewise, in reflection beams reflected from the fine line-and-space structure, the phase shift is caused and the reflectance spectrum is changed according to the variation of CD value. Accordingly, it may be possible to control the etching by directly measuring the dimensions of lines to thereby precisely control the CD value of lines in the etching by employing the scheme of the Japanese application supra.
To prevent the throughput from being lowered, however, it is necessary to provide an etching device with a monitoring device for measuring reflection beams in order to allow the measurement of the reflection beams to be performed for the etching device. Since the etching device has complex configuration, the monitoring device has limited mounting position. Accordingly, it may be difficult to install the monitoring device at a proper position where the measurements of the reflection beams from the fine line-and-space structure in a wafer can be optionally performed.
A semiconductor device, e.g., a chip, typically has two kinds of fine line-and-space structures as shown in FIG. 8. The sparse line-and-space structure having relatively large line pitch is a logic portion 82 and the dense line-and-space having relatively small pitch is a memory cell portion 83. Further, since the monitoring device may not be freely installed as described above, the monitoring device may not be placed close enough to the chip. Resultantly, the spot diameter of an emission beam from the monitoring device may become large, resulting in the emission beam irradiated both on to the logic portion 82 and the memory cell portion 83. Further, it may also be impossible to arrange the monitoring device to receive reflection beams reflected only from the memory cell portion 83. As a result, the monitoring device may receive the reflection beams reflected both from the logic portion 82 and the memory cell portion 83.
The method described in the aforementioned Japanese Application, is to measure the film thickness by detecting reflection beams reflected from a mask film having a single film thickness. In other words, the method does not consider a case of measuring film thickness by detecting reflection beams reflected from a mask film having various film thicknesses. As a result, if the method of the Japanese Application is applied to the chip in which reflection beams are reflected from both of the logic portion 82 and the memory cell portion 83, the spectral reflectance of a plurality of wavelengths of the received reflection beams is compared with a calibration curve prepared by considering detection of the reflection beams reflected from the single line-and-space structure. In such a case, however, accurate comparison may not be properly accomplished since the number of line-and-space structure for the detected spectral reflectance and that for the calibration curve are different. Accordingly, it is difficult to accurately control the CD value of lines in the line-and-space structures in the etching.